Welding backup shoes are employed at one side of a weld joint to prevent blowthrough of weld metal as welding is performed from the other side of the weld joint. In the welding together of pipe joints or lengths end-to-end, especially when pipes of relatively large sizes are welded together end-to-end, it is desirable when the welding is done from the pipe exteriors to provide a backup at the pipe interiors to prevent blowthrough of weld metal and to improve the uniformity and smoothness of the interior side of the weld so that grinding or other finishing operations of the weld at the interiors of the pipes will not be necessary. The interior side of the weld must be such that flow through the pipes will not interfere with and the weld must be formed with adequate weld penetration. Welding backups have, in the art, been provided by numerous forms of backup strips or shoes, none of which has been entirely satisfactory.
Weld metal blowthrough occurs most readily at the upper and lower sides of the pipes, and does not occur at the arcuately vertical pipe sides. One difficulty in providing backup apparatuses at the pipe interiors is that the backup apparatus is disposed between the shoes of a welding clamp device and must be capable of being expanded around the joint after the pipe ends have been brought together, and must be pressed against the interior pipe surfaces bridging the joint so that no lapses in backup will occur at least at the upper and lower sides of the pipes where blowthrough is most likely to occur. The invention seeks to provide improved backup apparatuses for use in welding pipes together end-to-end from the exteriors of the pipes so that satisfactory welds can consistently be obtained.